Reference is made to commonly owned, co-pending U.S. patent applications Ser. No. 12/793,398, filed Jun. 3, 2010, Ser. No. 12/793,470, filed Jun. 3, 2010 and Ser. No. 12/793,494, filed Jun. 3, 2010.
High intensity metal halide discharge lamps produce light by ionizing a fill contained in a discharge chamber of an arc tube where the fill is typically a mixture of metal halides and buffer agent such as mercury in an inert gas such as neon, argon, krypton or xenon or a mixture of thereof. An arc is initiated in the discharge chamber between inner terminal ends of electrodes that extend in most cases at the opposite ends into the discharge chamber and energize the fill. In current compact high intensity metal halide discharge lamps, the molten metal halide salt pool of overdosed quantity often resides in a central bottom location of the generally ellipsoidal or tubular discharge chamber, which discharge chamber is disposed in a horizontal orientation during operation. This is the coldest part of the discharge chamber during lamp operation and consequently is often referred to as a “cold spot” location. The overdosed molten metal halide salt pool that is in thermal equilibrium with its saturated vapor developed above the dose pool within the discharge chamber and is situated at the cold spot forms a thin film layer on a significant portion of an inner wall surface of the discharge chamber. This molten metal halide salt pool blocks or filters out significant amounts of emitted light from the arc discharge. The dose pool thereby distorts the spatial intensity distribution of the lamp by increasing light absorption and light scattering in directions where the dose pool sits in the discharge chamber. Moreover, the dose pool alters the color hue of light that passes through the thin liquid film of the dose pool.
Designers of luminaires and optical projection systems, and particularly of automotive headlamps associated with these types of lamps must consider these issues when designing the beam fowling optics. For example, distorted light rays are either blocked by non-transparent metal or plastic shields, or the light rays may be distributed in directions that are not critical for the application. These distorted light rays passing through the dose film are thus generally ignored and because of this the distorted light rays represent losses in the optical system since the distorted light rays do not take part in forming the main beam of the headlamps.
In an automotive headlamp application these scattered and distorted light rays are used for slightly illuminating the road immediately preceding the automotive vehicle, or the distorted rays are directed to road signs well above the road. Because of these beam collection losses, efficiency of the optical systems of automotive headlamps equipped with compact high intensity discharge lamps is typically no higher than about 40% to 50%.
As compact discharge lamps become smaller in wattage, and also adopt reduced geometrical dimensions, a solution is required with the light source in order to avoid such light collection losses in the optical system. This would result in achieving higher illumination levels along with lower energy consumption of the headlighting system.
Thus, a need exists to address the strong shading effect associated with the dose pool, and the impact on performance and efficiency of the headlamp optics designed around the lamp as a result of the uneven light intensity distribution from the lamp.